The traditional analog measuring instruments for measuring a water level in a boiler drum have a number of defects. Specifically, they cannot measure the water level in a boiler drum reliably and accurately over the whole working range. The specific analyses are shown as below.
When measuring a water level in a boiler drum under high pressure and high temperature condition, the traditional analog measuring instruments for measuring a water level in a boiler drum generally utilize a single-compartment balance container or an inner balance container for measuring a water level in a boiler drum which is based on the principle of differential pressure measuring.
A single-compartment balance container for measuring a water level in a boiler drum according to FIG. 1 comprises a cylindrical balance container (103), a steam side sampling pipe (102) welded to the left side of the balance container (103), and a positive pressure side outlet pipe (104) welded to the bottom of the balance container (103). A negative pressure side outlet pipe (105) led out from a water side sampling pipe (106) of a boiler drum (101) and the positive pressure side outlet pipe (104) are connected to a differential pressure transmitter (107). The differential pressure transmitter (107) sends an output signal through a cable (108) to a display indicator (109). One of the drawbacks of this design is that the water in the balance container (103) will transfer the heat along the positive pressure side outlet pipe (104), this will result in the water within the positive pressure side outlet pipe (104) having a higher temperature than its surroundings. The temperature difference will introduce an uncertainty to the density of the water within the positive pressure side outlet pipe (104) and the balance container (103), which could not guarantee the accuracy of the measurements (The water within the positive pressure side outlet pipe (104) and the balance container (103) is used as the reference for the measurements). The second drawback is that the water inside the boiler drum (101) is not saturated, the density is uncertain, thus the accuracy of the measurements could not be guaranteed. The third drawback is that under certain conditions (for example, during the starting or shutting off phase, during the water release in the event of pressure, or during boiler maintenance period), the water within the balance container (103) and the positive pressure side outlet pipe (104) which is the reference for the measurements could not be formed in an accurate and timely manner, thus the measurements might be influenced. The fourth drawback is that the clogging, leaking, and other environmental factors that effects the positive pressure side outlet pipe (104) and the negative pressure side outlet pipe (105) will result in inaccurate measurements.
An inner balance container for measuring a water level in a boiler drum according to FIG. 2 comprises a cylindrical condenser (204), a positive pressure side outlet pipe (205) welded to the right side of the condenser (204) and a steam side sampling pipe (203) welded to the left side of the condenser (204). The steam side sampling pipe (203) is connected to a condensation water inlet pipe (202) inside a boiler drum (201) and transfers the condensation water to the balance container (206) within the boiler drum (201). A positive pressure side outlet pipe (209) and a negative pressure side outlet pipe (208) are led out from a boiler drum water side sampling pipe (207) to a differential pressure transmitter (210). The differential pressure transmitter (210) sends out a signal through a cable (211) to a display indicator (212). The drawbacks of this design are that (I) under certain conditions (for example, during the starting or shutting off phase, during the water release in the event of pressure, or during boiler maintenance period), the water column within the balance container (206) and the positive pressure side outlet pipe (209) which is the reference for the measurements could not be formed in an accurate and timely manner, thus the measurements might be influenced. (II) Moreover, the clogging, leaking, and other environmental factors that effects the positive pressure side outlet pipe (209) and the negative pressure side outlet pipe (208) will result in inaccurate measurements.
In addition, the other types of analog measuring instruments for a boiler drum water level, such as a capacitive liquid level gauge, a ray liquid level gauge, and a magnetic liquid level gauge all have issues, such as issues with stability, maintenance, accuracy problem and the lackluster performing results.
A traditional capacitive liquid level gauge according to FIG. 3 comprises a boiler drum (301) and a measuring tube (305), and they are connected to each other through a steam side sampling pipe (302) and a water side sampling pipe (307). A capacitive sensor (306) inside the measuring tube (305) is connected to a transmitter (303) through a connecting flange (304). The drawbacks of this design are that (I) the capacitive sensor (306) is prone to scale formation, and it will produce a significant error in the measuring results. (II) The capacitive liquid level gauge is a U-tube type water level gauge, due to thermodynamic of the configuration, the dispersing of the heat leads to a lower temperature for the water inside the measuring tube (305) compared to the water inside the boiler drum (301). This will produce a relative significant error in the measurements.
A traditional ray liquid level gauge according to FIG. 4 comprises a boiler drum (401) and a measuring tube (405). The boiler drum (401) and the measuring tube (405) are connected through a steam side sampling pipe (402) and a water side sampling pipe (407). A measuring ray detector (406) within the measuring tube (405) is connected to a transmitter (403) through a connecting flange (404). The drawbacks of this design are that (I) the measuring ray detector (406) requires a radioactive source, and thus it is unsuitable for on-site configuration and operation. (II) The ray liquid level gauge is a U-tube type water level gauge, due to thermodynamic of the configuration, the dispersing of the heat leads to a lower temperature for the water inside the measuring tube (405) compared to the water inside the boiler drum (401). This will produce a relative significant error in the measurements.
A traditional magnetic liquid level gauge according to FIG. 5 comprises a boiler drum (501) and a measuring tube (505). The boiler drum (501) and the measuring tube (505) are connected through a steam side sampling pipe (502) and a water side sampling pipe (507). A waveguide sensor (506) is connected to a transmitter (503) through a connecting flange (504), and a magnetic float (508) is disposed on the waveguide sensor (506). The drawbacks of this design are that (I) the magnetic float is prone to magnetism loss under the high temperature condition. (II) The magnetic liquid level gauge is a U-tube type water level gauge, due to thermodynamic of the configuration, the dispersing of the heat leads to a lower temperature for the water inside the measuring tube (505) compared to the water inside the boiler drum (501). This will produce a relative significant error in the measurements.
In summary, there is a need to develop a new magnetic liquid level gauge for measuring a water level in a boiler drum which can have at least one of the following advantages: it can be used in the condition that the boiler drum are at high pressure and high temperature; it can be used under the full range of working conditions; it will be accurate, stable, reliable; and it can provides analog water level signals which can be used for water level adjustment and protection in the boiler drum.